Acaricidal Toxicity of Four Essential Oils, Their Predominant Constituents, Their Mixtures against Varroa Mite, and Their Selectivity to Honey Bees (Apis cerana and A. mellifera)

SIMPLE SUMMARY: Honey bees (Apis mellifera) that play vital roles in pollination and ecosystem maintenance, face severe threats from the ectoparasite, Varroa destructor. Existing control techniques, including mechanical, chemical, and organic, have had adverse effects on honey bees. Therefore, finding an easy, effective, affordable, and safe method is crucial. Essential oils (EOs) and their major components emerge as potential candidates due to their higher efficiency, biodegradability, and selectivity. However, evaluating composition variability, as well as their efficiency and safety in honey bee species, is essential. In this study, we assessed the efficiency of essential oil and their components against Varroa mites while studying the safety for honey bees. Eucalyptus globulus, Rosemary officinalis, Trachyspermum ammi (Ethiopian and Indian varieties), alongside their major components and a 1:1 mixture, were assessed for their acaricidal activity. All the samples exhibited acaricidal activity, with T. ammi, thymol, and the 1:1 mixture of thymol and carvacrol showing the highest efficiency against V. destructor. Importantly, the EOs and their major components showed selectivity and did not affect the honey bees’ learning and memory. In conclusion, our findings highlight the potential of T. ammi and the 1:1 mixture of thymol and carvacrol as candidates for Varroa control, suggesting further study at the colony level. ABSTRACT: The honey bee (Apis mellifera) faces a significant threat from Varroa destructor, causing the losses of millions of colonies worldwide. While synthetic acaricides are widely used to control Varroa infestations, excessive application has led to resistant strains and poses side effects on the host. Consequently, there is an urgent need for a new acaricide that is both effective and affordable, yet safe to use on bees. One potential source of these acaricides is essential oils (EOs) and their constituents. This study evaluated the acaricidal properties of four essential oils (Eucalyptus globulus, Rosemary officinalis, Trachyspermum ammi (Ethiopian and Indian varieties), their constituents and mixture of constituents against V. destructor through the complete exposure method. Our finding showed that a 1:1 mixture of thymol and carvacrol (4 h-LC(50) = 42 μg/mL), thymol (4 h-LC(50) = 71 μg/mL), and T. ammi oil (4 h-LC(50) = 81–98 μg/mL) were the most toxic test samples against V. destructor. Honey bee behavior and selectivity were also assessed with one additional EO Thymus schimperi, indicating that T. schimperi, T. ammi, and their components were selective and did not affect the learning and memory of bees. In conclusion, the thymol and carvacrol (1:1) mixture was shown to be a promising replacement for synthetic acaricides, being three times more toxic than a commercial acaricide, fluvalinate (4 h-LC(50) = 143 μg/mL).